The role of recycling in pulsed sputtering magnetrons

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1 The role of recycling in pulsed sputtering magnetrons Jón Tómas Guðmundsson 1,2,3, Daniel Lundin 3, Michael A. Raadu 1, Thomas J. Petty 3, Tiberiu M. Minea 3, and Nils Brenning 1 1 Department of Space and Plasma Physics, School of Electrical Engineering, KTH Royal Institute of Technology, Stockholm, Sweden 2 Science Institute, University of Iceland, Reykjavik, Iceland 3 Laboratoire de Physique des Gaz et Plasmas - LPGP, UMR 8578 CNRS, Université Paris-Sud, Orsay Cedex, France 70th Annual Gaseous Electronics Conference Pittsburgh, Pennsylvania November 9., 2017

2 Introduction Magnetron sputtering has been a highly sucessfull technique that has a number of industrial applications A magnet is placed at the back of the cathode target with the pole pieces at the center and perimeter The magnetic field confines the energetic electrons near the cathode The electrons undergo numerous ionizing collisions before being lost to a grounded surface

3 High power impulse magnetron sputtering discharge High ionization of sputtered material requires very high density plasma In a conventional dc magnetron sputtering discharge the power density (plasma density) is limited by the thermal load on the target High power pulsed magnetron sputtering (HPPMS) In a HiPIMS discharge a high power pulse is supplied for a short period low frequency low duty cycle low average power Gudmundsson et al. (2012), JVSTA Power density limits p t = 0.05 kw/cm 2 dcms limit p t = 0.5 kw/cm 2 HiPIMS limit

4 Ionization region model studies of HiPIMS discharges

5 Ionization region model of HiPIMS The ionization region model (IRM) was developed to improve the understanding of the plasma behaviour during a HiPIMS pulse and the afterglow The main feature of the model is that an ionization region (IR) is defined next to the race track The IR is defined as an annular cylinder with outer radii r c2, inner radii r c1 and length L = z 2 z 1, extends from z 1 to z 2 axially away from the target The definition of the volume covered by the IRM From Raadu et al. (2011), PSST

6 Ionization region model of HiPIMS The temporal development is defined by a set of ordinary differential equations giving the first time derivatives of the electron energy the particle densities for all the particles The species assumed in the of-irm are cold electrons e C (Maxwellian), hot electrons e H (sheath acceleration) argon atoms Ar(3s 2 3p 6 ), warm argon atoms in the ground state Ar W, hot argon atoms in the ground state Ar H, Ar m (1s 5 and 1s 3 ) (11.6 ev), argon ions Ar + (15.76 ev) titanium atoms Ti(a 3 F), titanium ions Ti + (6.83 ev), doubly ionized titanium ions Ti 2+ (13.58 ev) aluminium atoms Al( 2 P 1/2 ), aluminium ions Al + (5.99 ev), doubly ionized aluminium ions Al 2+ (18.8 ev) Detailed model description is given in Huo et al. (2017), JPD

Ionization region model of HiPIMS The model is constrained by experimental data input and fitted to reproduce the measured discharge current and voltage curves, I D (t) and V D (t), respectively Two

7 Ionization region model of HiPIMS The model is constrained by experimental data input and fitted to reproduce the measured discharge current and voltage curves, I D (t) and V D (t), respectively Two model fitting parameters were found to be sufficient for a discharge with Al target V IR accounts for the power transfer to the electrons β is the probability of back-attraction of ions to the target From Huo et al. (2017), JPD Experimental data from Anders et al. (2007), JAP

8 Ionization region model of HiPIMS A non-reactive discharge with Al target Current composition at the target surface From Huo et al. (2017), JPD Experimental data from Anders et al. (2007), JAP

9 Ionization region model of HiPIMS When the discharge is operated at 400 V the contributions of Al + and Ar + -ions to the discharge current are very similar At 800 V Al + -ions dominate the discharge current (self-sputtering) while the contribution of Ar + is below 10 % except at the initiation of the pulse From Huo et al. (2017), JPD Experimental data from Anders et al. (2007), JAP

10 Ionization region model of HiPIMS A primary current I prim is defined as ions of the working gas, here Ar +, that are ionized for the first time and then drawn to the target This is the dominating current in dc magnetron sputtering discharges This current has a critical upper limit 1 k B T g I crit = S RT ep g = S RT en g 2πm g k B T g 2πm g Discharge currents I D above I crit are only possible if there is some kind of recycling of atoms that leave the target, become subsequently ionized and then are drawn back to the target Anders et al. (2012), JPD Huo et al. (2014), PSST

11 Ionization region model of HiPIMS For the Al target the critical current is I crit 7 A The experiment is operated from far below I crit to high above it, up to 36 A. With increasing current I prim gradually becomes a very small fraction of the total discharge current I D The current becomes mainly carried by singly charged Al + -ions, meaning that self-sputter recycling or the current I SS recycle dominates From Huo et al. (2017), JPD Experimental data from Anders et al. (2007), JAP

12 Ionization region model of HiPIMS Recall that singly charged metal ions cannot create the secondary electrons for metal self-sputtering (γ SE is practically zero) The first ionization energies of many metals are insufficient to overcome the workfunction of the target material For the discharge with Al target operated at high voltage, self-sputter dominated, the effective secondary electron emission is essentially zero From Anders (2008), APL

13 Ionization region model of HiPIMS Reactive HiPIMS Ar/O 2 discharge with Ti target In the metal mode Ar + and Ti + -ions contribute roughly equally to the current combined self-sputter recycling and working gas recycling In the poisoned mode the current increaes and Ar + -ions dominate the current working gas recycling From Gudmundsson et al. (2016), PSST 25(6) (a) Discharge current contributions [A] (b) Discharge current contributions [A] I D total I D Ar + I D Ti + I D Ti 2+ I D O 2 + I D O + I D se t [ s] I D total I D Ar + I D Ti + I D Ti 2+ I D O 2 + I D O + I D se t [ s]

14 The generalized recycling model

15 Generalized recycling A working gas-sputtering parameter π g = α g β g ξ pulse where α g is ionization probability β g is back attraction probability ξ pulse = 1 is return fraction in a pulse The total current carried by working gas ions ( I g = I prim +I gas recycle = I prim 1 + π ) g 1 π g From Brenning et al. (2017), PSST

16 Generalized recycling The total self-sputter current is ( ) Yg π SS I SS = I g Y SS 1 π SS where the self-sputter parameter is π SS = α t β t Y SS The total discharge current is I D = I prim + I gas recycle + I SS ( = I prim 1 + π ) ( g 1 + Y ) g π SS 1 π g Y SS 1 π SS From Brenning et al. (2017), PSST

17 Generalized recycling The discharge current I D = I prim Π gas recycle Π SS recycl I prim is the seed current acts as a seed to the whole discharge current and has an upper limit I crit I prim Π gas recycle is the seed current for the self-sputter process If π SS > 1 the discharge goes into SS-runaway From Brenning et al. (2017), PSST

Generalized recycling Recycling map A graph in which the ion current mix of I prim, I gas recycle, and I SS to the target in a magnetron discharge is defined by a point The value of I prim /I D = 39

18 Generalized recycling Recycling map A graph in which the ion current mix of I prim, I gas recycle, and I SS to the target in a magnetron discharge is defined by a point The value of I prim /I D = 39 %, can be read on the diagonal lines (Y SS = 0.5) I prim /I D = 0.85 defines the dcms regime For I SS /I D > 0.5 we have the SS-recycle dominated range A For I gas recycle /I D > 0.5 we have the gas-recycle dominated range B From Brenning et al. (2017), PSST

Generalized recycling The discharge with Al target moves from the dcms regime to the HiPIMS discharge regime with increased discharge voltage type A A discharge with carbon

19 Generalized recycling The discharge with Al target moves from the dcms regime to the HiPIMS discharge regime with increased discharge voltage type A A discharge with carbon target jumps from the dcms regime to the HiPIMS regime both SS recycling and working gas recycling play a role intermediate type AB From Brenning et al. (2017), PSST

Generalized recycling Recycling map for five different targets with varying self-sputter yield Cu Y SS = 2.6 Al Y SS = 1.1 Ti Y SS = 0.7 C Y SS = 0.5 TiO 2 Y SS = 0.04 0.

20 Generalized recycling Recycling map for five different targets with varying self-sputter yield Cu Y SS = 2.6 Al Y SS = 1.1 Ti Y SS = 0.7 C Y SS = 0.5 TiO 2 Y SS = For very high self-sputter yields Y SS > 1, the discharges above I crit are of type A with dominating SS-recycling For very low self-sputter yields Y SS < 0.2, the discharges above I crit are of type B with dominating working gas recycling From Brenning et al. (2017), PSST

21 For high currents the discharge with Al target develops almost pure self-sputter recycling, while the discharge with Ti target exhibits close to a 50/50 combination of self-sputter recycling and working gas-recycling For very high self-sputter yields, above approximately Y SS 1, the discharges above I crit are of type A with dominating SS-recycling very little secondary electron emission little sheath energization of electrons For very low self-sputter yields, below approximately Y SS 0.2, the discharges above I crit are of type B with dominating process gas recycling significant secondary electron emission significant sheath energization of electrons. The fraction of the total electron heating that is attributable to Ohmic heating is over 90 % in the HiPIMS discharge

22 Thank you for your attention The slides can be downloaded at tumi/ranns.html and the project is funded by Icelandic Research Fund Grant No Swedish Government Agency for Innovation Systems (VINNOVA) contract no

23 References Anders, A., J. Andersson, and A. Ehiasarian (2007). High power impulse magnetron sputtering: Current-voltage-time characteristics indicate the onset of sustained self-sputtering. J. Appl. Phys. 102(11), Anders, A. (2008). Self-sputtering runaway in high power impulse magnetron sputtering: The role of secondary electrons and multiply charged metal ions. Appl. Phys. Lett. 92(20), Anders, A., J. Čapek, M. Hála, and L. Martinu (2012). The recycling trap : a generalized explanation of discharge runaway in high-power impulse magnetron sputtering. J. Phys D: Appl. Phys. 45(1), Bohlmark, J., J. T. Gudmundsson, J. Alami, M. Lattemann, and U. Helmersson (2005). Spatial electron density distribution in a high-power pulsed magnetron discharge. IEEE Trans. Plasma Sci. 33(2), Bradley, J. W., A. Mishra, and P. J. Kelly (2015). The effect of changing the magnetic field strength on HiPIMS deposition rates. J. Phys. D: Appl. Phys. 48(21), Brenning, N., J. T. Gudmundsson, M. A. Raadu, T. J. Petty, T. Minea, and D. Lundin, (2017). A unified treatment of self-sputtering, process gas recycling, and runaway for high power impulse sputtering magnetrons. Plasma Sources Sci. Technol. 26(12), Gudmundsson, J. T., N. Brenning, D. Lundin, and U. Helmersson (2012). The high power impulse magnetron sputtering discharge. J. Vac. Sci. Technol. A 30(3), Gudmundsson, J. T., D. Lundin, N. Brenning, M. A. Raadu, C. Huo, and T. M. Minea (2016). An ionization region model of the reactive Ar/O 2 high power impulse magnetron sputtering discharge. Plasma Sources Science and Technology 25(6), Huo, C., D. Lundin, J. T. Gudmundsson, M. A. Raadu, J. W. Bradley, and N. Brenning (2017). Particle-balance models for pulsed sputtering magnetrons. J. Phys. D: Appl. Phys. 50(35), Huo, C., D. Lundin, M. A. Raadu, A. Anders, J. T. Gudmundsson, and N. Brenning (2014). On the road to self-sputtering in high power impulse magnetron sputtering: particle balance and discharge characteristics. Plasma Sources Sci. Technol. 23(2), Raadu, M. A., I. Axnäs, J. T. Gudmundsson, C. Huo, and N. Brenning (2011). An ionization region model for high power impulse magnetron sputtering discharges. Plasma Sources Sci. Technol. 20(6),

Modélisation particulaire du plasma magnétron impulsionnel haute puissance

Modélisation particulaire du plasma magnétron impulsionnel haute puissance Tiberiu MINEA Laboratoire de Physique des Gaz et Plasmas LPGP UMR 8578 CNRS, Université Paris-Sud, 91405 Orsay Cedex, France tiberiu.minea@u-psud.fr